1. Field of the Invention
The invention relates to an ink composition.
2. Related Art
As image recording methods for forming an image on a medium to be recorded such as paper based on an image data signal, an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system and the like may be exemplified. For example, the inkjet system uses an inexpensive equipment and, since an image is formed directly on a medium to be recorded by jetting ink only on a required image area, the ink may be used efficiently and the running cost is low. Furthermore, the inkjet system makes little noise, and is excellent as an image recording system.
It is possible to print not only on plain paper but also on a non-water absorbing medium to be recorded such as a plastic sheet or a metal sheet by using the inkjet system, but achieving higher printing speed and higher image quality upon printing is an important issue, and the time required for drying droplets and curing after printing greatly affects the producibility of the printed product and the sharpness of the printed image.
As one inkjet system, there is a recording system using inkjet recording ink that may be cured by irradiation of radioactive ray. According to this method, producibility of printing is improved and a sharp image may be formed by irradiating with radiation immediately after jetting of the ink or after a predetermined period so as to cure ink droplets.
By achieving improvement of sensitivity of an inkjet recording ink which may be cured by irradiation with radiation such as ultraviolet ray, high curability against radiation is imparted, and many benefits including improvement of producibility of inkjet recording, decrease in power consumption, elongation of lifetime by decreasing load on a radiation generator, and prevention of generation of evaporation of low molecular materials due to insufficient curing, are obtained. Furthermore, improvement of sensitivity specifically improves the intensity of an image formed by inkjet recording ink.
Examples of preferable properties in an image formed by ink include adhesibility to a substrate and flexibility of the image. Specifically, flexibility of the image is an important factor when an image is formed on an uneven substrate or the surface of a flexible substrate such as a resin film, as well as when a printed formed article in which a formed article is formed after printing on the surface, such as a resin bottle, is produced, but it is difficult to balance flexibility with curability. That is, when the ratio of a monofunctional monomer is increased so as to improve flexibility, there is concern that curability is decreased, or tackiness of the image surface is caused due to bleeding of uncured low molecular components.
In order to improve adhesibility, flexibility and curing characteristics, a technique for using an urethane acrylate having low viscosity in an ink composition for inkjet has been suggested (e.g., see Japanese Patent Application Laid-open (JP-A) No. 2002-167537).
In order to obtain a photopolymerization initiator composition which cures with high sensitivity and is excellent in preserving property, a technique using a multifunctional thiol compound having a specific structure in a photosensitive composition as one of components in a photopolymerization initiator composition has been disclosed (e.g., see JP-A No. 2004-149755 and European Patent Application Laid-open No. 1983017).